Vegetation May Speed Warming of Arctic

Climate change will drastically change vegetation patterns in the Arctic, which will in turn spur additional warming, according to a new study.

The research, published yesterday in Nature Climate Change, outlines a counterintuitive side effect of climate change: As higher temperatures drive plants and trees into areas now inhospitable to them, their new distribution speeds up temperature rise via natural processes such as releases of heat-trapping water vapor into the air.

Previously, scientists understood that global warming would further "green" the Arctic but had less understanding of the degree and how these shifts would interact with the climate itself.

"The more greening you get, the more warming you expect," said Richard Pearson, lead author of the study and a research scientist at the American Museum of Natural History in New York City. The National Science Foundation funded part of the research.

The main driver of the projected warming is that trees and tall shrubs cover the landscape, making it dark, according to Michael Loranty, a study co-author and an assistant professor in the department of geography at Colgate University. Solar radiation is then more easily absorbed in these dark areas, rather than hitting snow and being reflected back into space.

The researchers considered three established climate models of temperature and precipitation into the 2050s. They considered a 30-year range, with the 2050s as the center, Pearson said. They then compared how 10 different vegetation classes, ranging from grasses to trees, would likely respond in the Arctic under these different future warming scenarios.

At least half of current vegetated areas are predicted to shift to a different type of vegetation class, with a general trend of now-present grasses and small shrubs yielding to larger shrubs and trees as the climate warms, the scientists said. The changes will be largely uniform throughout the Arctic, Pearson said.

"Smaller things will become bigger things, in effect," he said.

Forests and shade expand
Under the most aggressive warming scenarios, woody cover in the Arctic would increase by 52 percent. Tree lines would migrate more than 100 miles north, stretching into remote areas like northern Siberia.

Under the least aggressive climate models, woody cover still would expand a "significant" amount in the Arctic, or 12 percent, according to the study.

Using satellite data, the scientists then assessed how this new tree and plant cover would drive three climate feedbacks: water vapor in the air, carbon absorption by plants and the reflectivity of the Earth's surface.

Via natural processes, plants and trees release water vapor, which traps heat in the atmosphere. Shrubs and trees also make Arctic areas dark, allowing sunlight to be absorbed rather than reflected into space, creating additional warming via what is termed the albedo effect.

While plants also absorb carbon from the air, the team found that the warming power of water vapor and the albedo effect in particular far outweigh this cooling factor.

The cooling impact is "negligible" in comparison, Loranty said.

Pearson said it is too early to know how severe vegetation-driven warming could be. A prior study cited by the paper found that an increase in shrub cover of roughly 20 percent could spur as much as 1.8 degrees Celsius of additional regional warming over the next century.

"It's possible [the warming] would be more than that," Pearson said.

Greening already under way
Isla Myers-Smith, a lecturer at the University of Edinburgh who did not participate in the research, said the study is a "major advance" because it is the first to link such a broad range of vegetation changes with several climate feedbacks in the Arctic.

"The study provides more realistic modeling estimates of how much vegetation change will occur over the 21st century and will allow better predictions of future climate change," she said.

The research is also distinctive in that it considers 10 classes of plants and trees, rather than a few, said Song Feng, a climatologist at the University of Nebraska, Lincoln, who did not participate in the study.

Pearson said there already is extensive greening in the Arctic.

Another study this month reported that 32 to 39 percent of the Arctic experienced increased plant growth over the past 30 years (ClimateWire, March 11). Last year, scientists found that plants grew taller and evergreen shrubs increased at 46 Arctic locations between 1980 and 2010.

Pearson cautioned that the study does not mean definitively that a certain amount of tree cover will occur in the Arctic by 2050. It is possible vegetation expansions could move faster, or take longer, considering that there are some uncertainties about the migration patterns of plant species.

Widespread ramifications
A mountain range, for example, could delay the progression of trees into a certain area, even if the climate becomes suitable for them, he said. Another uncertainty -- not measured in yesterday's study -- is how plants and trees will interact with permafrost thaw.

There are recent studies showing that the shading effect of vegetation could also have a cooling effect by helping keep soils cool. That could delay permafrost thaw that further releases stored carbon, said Loranty.

But those prior studies considered small areas, he said. Other research signals that the albedo effect "causes so much warming that permafrost thaws even despite the cooling from shrubs," he said.

Myers-Smith said the interactions among plants and climate are extremely complex, and additional field work is going to be critical to fully understand what is happening in the Arctic.

The vegetation changes, when they do happen, could ripple through the ecosystem, said Pearson. Some bird species, for example, nest in polar regions and require open spaces to do so. More trees could disrupt their life cycles, he added.

"Widespread distribution of Arctic vegetation would have impacts that reverberate through higher tropic levels, affecting wildlife and ecosystem services that are important for human well-being, including food production, access to natural resources, and traditional cultural identity," the study says.

Clarification: This story was updated to clarify the driver of the albedo effect.